<script type="text/javascript">

$('.show_more').on('click',function (e){
    $.ajax({
        type:'POST',
        url:'gallery_controller.php',
        success:function(html){
            $('.content').append(html);
        }
    });
});

<form action="" method="post">
<button type="submit" class="show_more" name="next">Next</button>
</form>

<?php
$servername = "localhost";
$username = "root";
$password = "";

$conn = new mysqli($servername, $username, $password, "dbgallery");

if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}

$result = $conn->query("SELECT imagefile FROM tbl_images ORDER BY id DESC LIMIT 10 OFFSET 0");

while($row = $result->fetch_assoc()) {

echo '<h1>'.$row["imagefile"].'</h1><hr />';

}
?>